Ce:LuPO4 (Ce:LOP) is a rare earth orthophosphate (REOP) crystal, whose intense emission, short fluorescent lifetime, and high positron absorption cross section could lead to more efficient and cost effective positron emission tomography (PET) instruments. While small research crystals can be obtained routinely by cooling a lead-based flux, large crystals suitable for incorporation into PET instruments are not readily grown. However, by introducing a proprietary, environmentally benign solvent developed to replace the lead-based flux, we believe high temperature solution (HTS) growth or top seeded solution growth (TSSG) could be the key to commercial production of large Ce:LOP crystals. The temperature and composition range for phase stability of Ce:LOP was investigated as part of the Phase I research program in order to define a robust production technology. Since the new solvent system can be applied to the growth of all REOP crystals, some of which have application as ultraviolet, visible, and infrared lasers, a commercial source of REOP crystals could ultimately lead to other new or improved medical instruments. The partial phase diagram derived from the temperature/composition study shows that Ce:LOP can be grown over a wide temperature range. The identified solvent demonstrates a high solubility with a temperature-dependent solubility gradient that will support HTS and/or TSSG. PROPOSED COMMERCIAL APPLICATION: Ce:LOP scintillator crystals can be utilized for the development of highly sensitive PET instruments due to a fortuitous combination of high density, high light yield, and fast decay time. A growth process developed for Ce:LOP crystals would apply broadly to all REOP crystals that have potential applications as ultraviolet, visible, and infrared lasers, as well as polarizers and optical isolators in laser systems.